Derek's Mystery Girl
by JustMe133
Summary: When Derek gets a secret admirer note with a request to meet, will he be ready for who's hiding behind the mask? ONESHOT. Dasey. If you don't like step-cest, don't read. Rated T for safety.


**So, just a really random idea I had and figured, hey, why the hell not post it?  
What else do I have to do with my free time now?**

**No, I don't own Life With Derek, nor the characters or any of that good stuff. Just a fan. No copyright intended.**

…

"Why did the school want to have an 'end-of-year-costume-bash' anyways? It was stupid. May isn't even _close_ to Halloween!" Derek continued to grumble to himself as he stood in front of the bathroom mirror and fixed his tie. "Stupid secret admirer wanting me to go." He stopped and threw his hands down, head hanging.

Why was he letting an anonymous person, who he assumed/hoped was a girl get to him? He pulled the note out of his pants pocket and read it once again.

'_Derek, I've wanted to speak up about my feelings for a long time, but I've been too shy. Wear this mask to the dance and I'll come to you. I'm ready. XOXO'_ He looked at the plain white mask that was left with the note. Why did he want to do this so badly?

Oh right, because he was trying to block out any feelings he might be feeling for the girl who happened to have the room right next to his; because he totally didn't need to be thinking of his _stepsister_ that way. With a sigh, he rolled up his sleeves and looked at himself.

A white dress shirt covered his upper body with the sleeves rolled up to his elbows, covered up by a black vest and black tie. He wore plain black slacks and black dress shoes.

"Not too shabby," he muttered, slipping the mask on and eyeing himself. "This is ridiculous." He ripped the mask off and put it in his pocket along with the note and the ticket to the dance. With a deep breath, he left the bathroom and, instead of heading to his room, knocked on Casey's door. "Hey Space Case, you want a ride to the dance or not?" Casey opened her door and eyed him. He was surprised to find her hair in two braids and a robe covering her. She crossed her arms and glared at him.

"I'm not going," she stated, eyes cool as she stared at him. "But you look spiffy. Have fun." She shut her door in his face, surprising him. He shrugged and turned before he yelled at her.

"Not like I wanted you in my car anyways!" Stomping down the stairs, he grabbed his keys and left, letting the door slam behind him.

….

As he stepped into the school's hallway, he could see the lights flashing and hear the music blasting from the gym, where the dance was being held. Groaning, he pulled the mask on and headed in.

No one seemed to know who he was, and for that, he was grateful; he leaned against and wall and just watched, waiting.

He hoped this would be worth the wait.

About 20 minutes passed before he saw someone look his way as they walked in with a couple of people. He felt his heart stop as she stared at him. He took in her outfit from the bottom up as she began to make her way to him. He wore black pumps –at least that's what he thought the shoes were called- ,black lacey leggings, a tight dark blue dress that had a lacy top, and, from what he could see, back, with black lacy gloves on her hands. Her mask was dark blue like the dress, covered with black lace as well. Her brown hair was wild about her shoulders, falling in curls around her face. Her lips were full and covered with a pink gloss that made him gulp as she continued to walk towards him. When she was finally in front of him, he saw the most gorgeous blue eyes he could ever imagine staring at him, surrounded by black eyeliner and blue eyeshadow.

"You came," she said softly, grinning at him; he thought she looked very catlike with that movement. He didn't say anything as she eyed him. "Derek. I'm guess you got my note," she said, raising a hand to gently tug on the mask that covered the lower part of his face. He found himself gulping as he caught the scent of her perfume.

It was intoxicating to him, just like her.

"Who are you?" She smirked now and reached for his hands.

"That's for me to know, and you to find out," she whispers, her voice husky and warm, and dear God Derek feels like he's melting into a puddle. "Come dance."

"I don't dance," he muttered even as she began to pull him out onto the dance floor.

"Tonight, you will." She challenged him, making him like her even more and more. As they began to move, he watched her move; she was lithe, her movements smooth and cunning. The more he watched, the more he thought of her as a cat; she gripped his hand and pulled him closer, her blue eyes watching him intensely.

"Tell me who you are," he said this time, more conviction in his words. She giggled and shook her head, making her brown wild curls shake as her lips curled up in a smirking grin once again.

"Not yet," she whispered into his ear, running a hand down his cloth-covered shirt. "You don't need to know yet."

Suddenly, she was gone. Derek blinked, realizing they had danced back to the corner he had been in before. He looked for her, but the gym was crowded and he couldn't see his mystery girl anywhere.

Where had she gone off to?

"Looking for me?" a teasing voice purred in his ear, making him turn to find her leaning against the wall, her eyes locked on him. He moved towards her, pinning her against the wall.

"You're sneaky aren't you?" She giggled and placed a hand against his chest as if to push him away.

"Only when it comes to you." Her voice is still husky; he suspects that it isn't her real voice she's using, just a mask to cover her up from him.

"I know you." He says it as a statement, not a question. She just shrugs and leans a little closer.

"Maybe you do, maybe you don't."

"What happened to the too shy girl who wrote me that note?" She laughed now, a bright and joyful sound that shook him to his core.

He had heard that laugh before.

He just couldn't put _where_ though.

"She's hiding while I play," she murmured, moving her face close to his. "Let me have my fun tonight Derek. Let me enjoy you while I can."

"Why can't you?" he asked, feeling confused. This girl was intoxicating, playful, and gorgeous. Something had to be wrong with her. She shook her head, her hand curling against his chest. He watched as worry flashed briefly in the blue orbs before it returned to playful.

"You wouldn't like me if you knew who I was. So, for tonight, let me just be your dream girl. Please?" Her words are still warm, but he can hear the longing underneath them.

"Okay," he finds himself saying, not regretting it because of the grin that spreads across her face.

"Thank you," she whispers, pressing her lips against his in a lust-filled kiss. He wraps his hands around her waist, pulling her closer. She giggles against his lips and pulls away. "There's time for that later. Come dance with me." He nods and she tugs him away, her hand clasped in his.

They danced the night away; fast dances that made her twirl in his arms and slow dances, her lean body pressed into his.

He realized, as she pulled away from him and led him back to their corner, he didn't want this night to end.

At least not until he knew who she was.

He slowly raised a hand to cup her face; she let him, eyes slipping closed at the gentle touch. He moved his hand up slowly, his fingers edging up under the mask a bit.

"No," she says, eyes flying open to reveal panic and her hand becoming grip-like on his wrist. "Derek please."

"Who are you?!" He was more urgent now; he didn't do love, but God, this girl got under his skin. She frowned and let go of him, stepping away.

"I'm nobody Derek. That's all." Her voice was a sad whisper, all the playfulness gone; he knows he ruined it, and moves closer.

"Fine. Don't tell me who you are," he says, turning her to face him; she won't meet his eyes this time. "But I want you to know, you're not nobody." She looked at him now, blue eyes wide. "I… I don't do _love_, or _feelings_, but you… you sparked something in me." She smiles softly and cups his face.

"You don't know how badly I want you Derek," she murmurs, pressing her lips to his in a feverish kiss. "But we can't be together. I just wanted you to finally know how I've been feeling." They continued to kiss, Derek holding onto her like his life depended on it. His fingers itched to rip the mask off and find out who the girl was, to know who held his heart.

But the heartbreak and panic in her eyes from before stopped him from doing it.

Finally, they broke apart for air.

"I have to leave," she said softly, her hands tangled in his hair.

"No. Stay here with me." She smiled sadly and cupped his face.

"You don't know how much I wish I could. Goodbye Derek." She gave him one last kiss before she was gone, leaving him alone in the still-crowded dance.

He had never felt so empty before.

…

Derek got home about 30 minutes after she had left; he had stayed a little longer, hoping she would reappear, but she didn't. He had finally given up and gone home.

He knew everyone was asleep, so he just went to his room and fell onto his bed, ripping the mask off and throwing it onto the floor.

That girl had rocked his world and he hadn't expected that. He brought a hand up to his lips, still able to feel her lips against his.

He fell asleep that way, dreaming of the blue eyes he'd never see again.

…

Derek woke up late the next day, his heart breaking again as he remembered the girl from the night before. She had shaken him in ways no one ever had. Groaning, he stripped out of his fancy clothes and put on regular ones, trumping down the stairs.

Casey was the only one in the kitchen, reading a book and sipping a glass of water.

"Morning Sleepyhead," she said, not looking up. He grunted at her cheerfulness and grabbed the milk. "You okay?" He chugged the milk and put it back, plopping into the seat across from her.

"Of course Space Case, why wouldn't I be?" he said, nonchalant as he could be. She eyed him for a minute before speaking.

"How was the dance last night?" At the mention of it, his mood darkened. "That bad?"

"No… it was…" he shook his head and stopped talking.

"Derek." He jerked at the voice, staring at Casey. "What is it?"

"N-Nothing," he muttered, watching her. Her voice, when she had said his name – or had he imagined it?- sounded so much like the girl from last night.

"Is there anything you want to talk about? You seem… on edge." He shook his head and stood up.

"Nah Case. I'm gonna go for a walk." She watched him grab his leather jacket and practically run out of the house to get away from her. She closed her book and shook her head at his antics before heading up to her room.

…

Derek found himself at a small coffee shop he didn't even know existed. He sat on one of the outside tables and closed his eyes, only seeing the intense playful blue ones from last night.

"You look like you need some company." He jerked up, his mouth falling open as she smiled at him.

It had to be the girl from last night; she wore the same black heels, but her legs were bare, covered only by a short skirt. His eyes traveled farther as he saw a tight shirt covered her body. Her hair was just as wild and curly as before, but instead of a mask, there was a large pair of sunglasses that dominated her face.

"You."

"Me. Can I sit?"

"Of course!" She sat across from him, smiling at him as she perched her chin on her hand.

"I told myself that I needed to stay away from you," she murmured, watching him; at least, he thought she was; he couldn't her eyes.

"Why would you do that?" he asked, turning back into charming Derek. She giggled and moved her chair a little closer to him.

"You're bad for me Derek," she said, tracing a finger over his hand. "We… we're bad for each other. That's why I was so shy. I know how this will end. But… I can't seem to stay away."

"Why would we be bad for each other?" She shook her head and raised a hand to run it down his cheek.

"You don't want to know." He grabbed her hand and kissed it, making her blush.

"Der-ek," she giggled, breaking his name into two syllables. It crashed into him, making him shake; the word was too familiar. He shook his head while she smiled at him.

_No way_.

"What is it?" she asked, cocking her head to the side some, making the brown curls bounce off her shoulder.

"Nothing," he murmured, smiling. "I just… I want to be with you."

"No you don't. Trust me." She leaned over and kissed him then, making his chest ache as she pulled away.

"No," he said, pulling her closer and kissing her again, surprised by how desperate he felt for her to be close to him; he had just met her yesterday, but he already knew he was in love.

And Derek didn't do love.

"I have to go," she whispered, her breath hot on his lips. "Don't forget about me."

"Never." She giggled and was gone, just like the night before. Groaning, he began the trek home.

…

"Hey, you okay?" Derek jumped, not knowing Casey was in the living room. She looked at him expectantly, her eyes boring into him. He stopped to stare at the blue orbs, finding himself lost in memories. "Derek? What is it?" He moved towards her and sat down on the couch with her instead of his armchair.

"I'm in love," he muttered, making her gasp and close the book she had been holding.

"What?! With who?"

"I don't know!" he cried, finally breaking; who better to talk feelings with than a girl? "I've only known her a matter of two days, and know literally nothing about her, but I… I can't get her out of my head. It feels like I've known her longer. And when she's not here, in my arms… I feel… empty. I _need_ her, but she said we can't be together. I miss her. And I have no fucking clue who she is." Derek stopped talking and looked at Casey, who was staring at her book, lost in thought. "Space Case?" She shook her head and jumped up, bolting up the stairs. "What the fuck?" he asked, chasing after her. He banged on her door, but no response. "When did you install a lock?" he wondered, digging in his wallet for his library card – which he never used, just had- and began to wiggle it in the crack in her door, finally jiggling it open. He stepped in and found Casey on her bed, not looking at him.

"Go away Derek." The voice she spoke with was warm and hurt, and it was a slap to the face. He closed her door behind him and stepped farther into the room.

He stopped when he saw something on her desk. Picking it up, he looked at the object in his hands.

A blue-black-lace-covered mask.

"You?" he finally got out, turning to look at the girl who was huddled on her bed. "Case…"

"You weren't supposed to fall in love with me," she muttered, sitting up to look at him, blue eyes hard. "I just wanted one night where we weren't stepsiblings and were just two teenagers." She stood up and pulled her hair down from the clip it was in; he watched as her normally straight hair fell in curls around her face. "I didn't expect you to actually _like_ me."

"Case…"

"I was content with only last night, but you seemed so… upset this morning that I just had to see you again. But then… you came home and told me… Derek." Her voice, instead of the high-pitched whine he was used to, was the warm voice of the girl from before.

"What happened to your voice?" she laughed that laugh again and shook her head.

"I began to talk higher to make sure you never caught on to how I felt. I'm guessing it worked."

"Yeah… Casey… why didn't you tell me?"

"You know why Derek. Whatever you felt for me… for the girl from the dance, you're just going to have to forget about it." She took the mask from his hands and frowned. "I'm sorry I did this. It was a horrible idea." She kissed him then, as herself, before she pushed him out of her room. "Goodbye Derek."

Derek stood there in the hallway, hearing her lock click.

He didn't try to open it this time.

…

They passed by each other in a daze, ignoring each other.

There were no more loud teen arguments, no more screams of annoyance, no more snarky comments.

Nothing but silence between the two of them.

Derek watched her every day, making her permanently on edge and clumsier than usual.

He didn't care now though.

He knew he wanted her, and that she wanted him too.

But how could he make her see that?

He sighed and went to the living room, waiting for the opportunity to present itself.

"You okay son?" Derek hadn't realized his dad and Nora were in the room when he had gone to sit in his chair.

"Oh… yeah."

"It's two weeks into summer break. You and Casey have been civil lately. Why?" Nora asked, making him shake his head.

"No reason. I guess we grew up."

"Derek." He heard Nora say his name and it reminded him of Casey.

"_I know how this will end_." Casey's words echoed in his mind and he stood up, facing his dad and stepmom.

"I'm in love with Casey." The words were out of his mouth before he could realize what he was saying. The two adults stared at him, surprised.

"What?" Nora finally asked, and Derek found himself breaking.

"I am in love with your daughter. I want to be with her. And I know she wants to be with me too. But we can't because… we're family." Nora looked at George, who was staring at his son still, mouth slightly opened. Nora stood up and walked out of the room, leaving the two Venturi men alone. "Dad?"

"When… did this happen?"

"About two weeks ago."

"Derek… you know you and Casey can't be together. I mean, Nora and I were a bit worried in the beginning, you two being so close in age, but when you two hated each other on site I felt a bit better. But now…"

"I'm sorry Dad. But you asked."

"Derek." He turned to find Nora entering the room, a small box in her hands. "If you… If you mean what you say, that you want to be with my daughter…then here." He opened the box, looking at was inside. "It's been in the family for a little while. Casey will recognize it." Derek looked at her, surprised.

"You mean… you don't mind?"

"Derek, as a woman, I can see the love you feel for Casey in your eyes. As her mother, I want her to happy. As your _step_mother, I want _you_ happy. If with each other is the way to accomplish that… just don't hurt her."

"Never." She smiled and gave his arm a squeeze before they looked at George, who stood up slowly.

"Nora… are you sure?"

"Yes. He loves her like you love me. You can't stop love like that George. Even if they're stepsiblings." George looked at his wife and his son before he nodded.

"Okay then." Derek grinned at the both of them before he ran up the stairs and barged into Casey's room, surprising her.

"D-Derek," she whispered, looking at him. "What do you want?"

"You," he said simply, pulling her up to her feet. "I love you. I haven't stopped in the two weeks you've avoided me. And… I promise you it'll be okay."

"How can you promise that? Our family…" he voice trailed off as he opened the box and put it into her hand. "That's my mom's…"

"She gave it to me. To give to you." Casey looked at him, eyes burning.

"You told her…"

"And my dad." Casey's mouth opened in shock as she looked at the box. It was ring that had been her mom's, and her mom's before that.

"Derek…" she whispered, tears bubbling up. "You mean…"

"Think of it as a promise ring for now." He slipped it onto her finger, grinning when it fit perfectly. "I love you Space Case."

"Der-ek," she said weakly, making him chuckle as he pulled her close. "You're… serious."

"Yep. My mystery girl." She flushed at that and grinned as he moved closer, finally kissing her in the way he had wanted to for so long now.

"I love you."

…

**So yeah… this was uber long. Maybe I should've broken it up.**

**Oh well.**

**Sorry if the characters were OOC, that happens when you haven't seen a show in a while.**

**I hope ya'll liked it though and thanks for reading!**

**-JustMe133**


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